The invention relates in general to fuel systems in internal combustion engines and in particular to high-pressure direct injection fuel systems.
The present invention is an improvement to the invention disclosed in U.S. Pat. No. 5,832,900 issued Nov. 10, 1998, entitled "Fuel Recirculation Arrangement and Method for Direct Fuel Injection Systems" and having the same inventor as the present application. The aforementioned U.S. patent is hereby expressly incorporated by reference.
Fuel injection systems have in recent years been equipped with injector valves which protrude from pressurized fuel rails, with fuel sprayed out of the injector valves when each is opened at timed intervals by the engine controls. The injectors typically have been arranged to spray the fuel into the intake manifold adjacent the intake valves of the engine cylinders such that the fuel in the fuel rail need only be pressurized to moderate levels, i.e., 3-4 bars.
In "direct injection" systems the injectors spray fuel directly into the engine cylinders. This requires a much higher fuel pressure, on the order of 20-140 bars at the injector tip. The fuel pressure is developed by a high-pressure pump. Fuel is supplied from the fuel tank to the inlet of the high-pressure pump by a conventional lower pressure supply pump.
As engine load varies, fuel demand is varied. To meet changing fuel demand, the pressure in the fuel rail is controlled by a regulator allowing a controlled outflow of fuel from the fuel rail to a lower pressure region. The high operating pressures in the fuel rail create the possibility that the fuel will be changed to a gaseous state when discharged by the regulator. Lighter dissolved components of the fuel, such as butane, methane, or even air, have a tendency to separate and form bubbles, which are very difficult to recombine with the liquid fuel. The presence of bubbles in the liquid fuel recirculated back to the high-pressure pump inlet could damage the high-pressure pump.
The work performed by the high pressure pump in raising the fuel to these high pressures also adds to the internal heat of the fuel, increasing the tendency for gaseous formation of bubbles to occur.
U.S. Pat. No. 5,832,900 (the '900 patent), referenced above, provides a fuel recirculation arrangement and method for minimizing the tendency for fuel to gasify in such high-pressure direct injection systems. In the '900 patent, the high-pressure return fuel flows through a fixed size orifice. The fixed size orifice limits the operating range of the recirculation system. At low flow rates of the high-pressure return fuel, most of the high-pressure pump output is being used by the flow to the injectors. Thus, at low flow rates of the high-pressure return fuel, it is difficult to maintain a minimum backpressure in the supply line to the high-pressure pump.